1. Field of the Invention
The present invention is directed to a coin tone validator (CTV) for verifying coin drops occurring in a telephone paystation and, more particularly, to a CTV located in a local central office and connected between a pay telephone and a switching system and/or voice message storage system. The CTV monitors DC loop current changes in addition to measuring standard 1700 Hz and 2200 Hz coin tone frequencies generated when a coin is dropped.
2. Description of the Related Art
When a caller places a call, a coin is dropped into a pay telephone. The coin must be physically validated before the coin amount is determined. If the coin is determined to be counterfeit, it is returned to the caller by way of a coin return slot. If the coin is determined to be real, the coin is inserted into a coin signalling unit. Upon receipt of the coin, the conventional coin signalling unit switches off the mouthpiece of the handset so that noise contamination is eliminated. A coin tone oscillator is then placed on the line and sends appropriate coin tone signal bursts down a loop to a local central office. After the coin tones have been sent, the mouthpiece of the handset turns on and the coin is directed into a coin relay hopper. At this time, an electrical connection is made, via a coin relay, from a TIP side of the loop to EARTH to provide a signal to satisfy a coin presence test to be performed by the local central office. (The TIP side being a physical tip of a cord board connector; a RING being a slip ring around the cord board connector, and a sleeve being EARTH). A coin call cannot be completed without this connection being established. Coins in the coin relay hopper remain there until the call is terminated by the caller hanging up. At that time, the coins are released into either a coin collection box or a return slot if the call was not completed. The TIP to EARTH connection is removed at this time.
In pay telephone systems, when the handset of the telephone is first lifted from the cradle causing an "off-hook" state and initiating a dial tone, coin tones are not generated from the coin signalling unit until enough coins are inserted into the pay telephone to meet a base rate. The base rate is the minimum amount of coinage needed to initiate a coin call. The base rate is set internally in the pay telephone. For example, if the base rate of the pay telephone is set to twenty cents, the coin signalling unit will not generate any coin tones for five, ten or fifteen cents dropped into the pay telephone. When, however, twenty cents or more has been inserted into the pay telephone, the total amount inserted is sent in bursts by the coin tone oscillator to the local central office. Each burst represents five cents.
The majority of pay telephone sets in use are C-type sets and D-type sets; the D-type set being a newer version of the C-type set. In C-type sets when a coin is dropped, a coin tone signal burst is output from a coin tone generator for every five cents a caller inserts (a quarter consists of 5 consecutive tone bursts). The coin tone generator continuously generates coin tone bursts which have an immediate loop current drop at the beginning of the coin tone signal burst string and then a loop current surge upon completion. Tones appear during the loop current drop. These coin tone signal bursts must conform to the "tone-on", "tone-off" times specified by Bell Communication Research (Bellcore) or will not be accepted by the central office tone detection equipment as valid tones. In a D-type telephone set, a separate loop current pulse is present for each coin tone burst caused by a coin being dropped into the pay telephone. Each of the loop current pulses exhibits a fast rise time but slow decay time.
There is a problem, however, in that pay telephone users can fraudulently generate coin tones from the calling station. External devices can be used to inject the proper coin tone frequencies through the mouthpiece of the handset once one real coin has been inserted into the pay telephone to establish the TIP to EARTH connection. Prior art detection systems determine only whether the frequencies generated by the coin tone generator conform to Bellcore specifications, before performing a coin presence test at the end of a digit string. As a result, the central office equipment cannot distinguish between real coin tone energy generated by the coin tone generator and injected signals of the same frequencies. Therefore, an accurate validation of the correct amount of coins deposited is not possible with the prior art detectors.